Field of the Invention
The invention relates to a thermochromic ink composition intended more particularly for cards formed of a plurality of layers of at least one polymeric material—especially of polycarbonate—which are hot laminated under pressure, for synthetic papers and for security papers.
Description of the Related Art
EP0400220 describes a laminated article for identification purposes (such as an identity card) comprising two sheets or layers of PVC, at least one of which is transparent, between which there is laminated a layer comprising thermochromic liquid crystals arranged on a contrasting dark base. The liquid crystals are applied in the form of a water-based ink comprising a water-soluble polymeric binder, for example gelatin or a polyacrylamide. As indicated by this document, hot lamination under pressure of sheets of PVC bearing information causes adhesion problems. This document provides a cleaning step prior to lamination, with the aim of eliminating the chemical compounds which are assumed to impair the adhesion of the various layers to one another. Nevertheless, this article has not been exploited in practice.
Various other attempts have been made to incorporate thermochromic motifs into cards formed of a plurality of thermoplastic layers which are hot laminated under pressure. However, the known thermochromic ink compositions which are envisaged especially in these applications are compositions comprising an aqueous solvent. EP2322587 describes an example of a thermochromic ink composition which comprises microcapsules incorporating a thermochromic composition and UV absorbers dispersed in an aqueous solvent with a polymeric, for example acrylic, binder.
Moreover, a large number of cards which form data carriers and are used for identification or security purposes (bank cards, identity cards, driving licenses, cards for individual access to shared sites or transport, passport pages, etc.) are formed by hot lamination under pressure of a plurality of layers of thermoplastic material—especially of polycarbonate, which material moreover has many advantages in such applications (rigidity, durability, ease and reliability of marking (various inscriptions, security motifs, etc.), possibility of laser marking, etc.). However, the inventors have found that the thermochromic ink compositions comprising an aqueous solvent which have been proposed hitherto are not compatible with production methods by hot lamination of such layers of thermoplastic material, and in particular with the hot (temperature of approximately from 150° C. to 200° C.) lamination under pressure (approximately 2×104 Pa) of layers of polycarbonate. Such laminating conditions in fact destroy the thermochromic effect obtained by the thermochromic ink compositions printed on the layers of polycarbonate. Furthermore, problems of adhesion of the polycarbonate layers with one another and with the printed thermochromic compositions are irremediably found. Likewise, the known thermochromic ink compositions have a limited lifetime of at best one or two years, which is insufficient in the case of identification or security applications, for which a lifetime of at least 10 years is required (see especially ISO 24789-1,2 (2012)—“Method of evaluation of identification (ID) card service life”). The same problem arises with the synthetic papers and security papers that are used in security applications (authentication and/or anti-forgery) and/or identification applications, which papers are often hot laminated under pressure.
It is to be noted in this respect that thermochromic ink compositions which cure by ultraviolet radiation are also not compatible with hot lamination methods, the thermochromic activity being destroyed under the action of the heat and/or over time, and produce a far less reactive thermochromic effect which is insufficient in the above-mentioned applications in which the thermochromic components are isolated thermally from the outside by at least one layer, especially a layer of polycarbonate.
In addition, polycarbonate, synthetic papers and security papers are sensitive to most organic solvents and are therefore not compatible with printing using inks based on organic solvents.
Consequently, the suggestions which have been made of incorporating thermochromic motifs into cards—especially of polycarbonate—which are hot laminated under pressure, synthetic papers and security papers as yet constitute only theoretical teaching which cannot be put into practice on an industrial or commercial scale, in particular for producing data carriers which can be used for identification or security purposes.